Bump-on-trace interconnect

ABSTRACT

Disclosed herein is a bump-on-trace interconnect with a wetted trace sidewall and a method for fabricating the same. A first substrate having conductive bump with solder applied is mounted to a second substrate with a trace disposed thereon by reflowing the solder on the bump so that the solder wets at least one sidewall of the trace, with the solder optionally wetting between at least half and all of the height of the trace sidewall. A plurality of traces and bumps may also be disposed on the first substrate and second substrate with a bump pitch of less than about 100 μm, and volume of solder for application to the bump calculated based on at least one of a joint gap distance, desired solder joint width, predetermined solder joint separation, bump geometry, trace geometry, minimum trace sidewall wetting region height and trace separation distance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/997,124, filed on Jun. 4, 2018, entitled “Bump-on-TraceInterconnect,” which is a continuation to U.S. patent application Ser.No. 15/065,632, filed on Mar. 9, 2016, entitled “Bump-on-TraceInterconnect Having Varying Widths and Methods of Forming Same,” nowU.S. Pat. No. 9,991,224 issued on Jun. 5, 2018, which is a divisional toU.S. patent application Ser. No. 13/653,618, filed on Oct. 17, 2012,entitled “Bump-on-Trace Interconnect,” now U.S. Pat. No. 9,299,674issued on Mar. 29, 2016, which is related to, and claims priority toU.S. Provisional Application No. 61/625,980, titled, “SemiconductorDevice Package” filed on Apr. 18, 2012, which applications are hereinincorporated by reference in their entireties.

BACKGROUND

Generally, one of the driving factors in the design of modernelectronics is the amount of computing power and storage that can beshoehorned into a given space. The well-known Moore's law states thatthe number of transistors on a given device will roughly double everyeighteen months. In order to compress more processing power into eversmaller packages, transistor sizes have been reduced to the point wherethe ability to further shrink transistor sizes has been limited by thephysical properties of the materials and processes. Designers haveattempted to overcome the limits of transistor size by packaging everlarger subsystems into one chip (systems on chip), or by reducing thedistance between chips, and subsequent interconnect distance.

One method used to reduce the distance between various chips forming asystem is to stack chips, with electrical interconnects runningvertically. This can involve multiple substrate layers, with chips onthe upper and lower surfaces of a substrate. One method for applyingchips to the upper and lower side of a substrate is called “flip-chip”packaging, where a substrate has conductive vias disposed through thesubstrate to provide an electrical connection between the upper andlower surfaces.

Solder ball grid arrays are also a technique sometimes used to joiningpackages, with an array of solder balls deposited on the bonding pads ofa first package, and with a second package joined at its own bonding padsites to the first pad via the solder balls. The environment with thesolder ball grid array is heated to melt the solder balls and thepackages compressed to cause the solder balls to contact the pads onboth packages.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIGS. 1 and 2 are cross-sectional diagrams of embodiments of a BoTinterconnect element;

FIG. 3 is an enlarged cross-sectional illustration of a BoTinterconnect;

FIG. 4 is cross-sectional illustration of multiple BoT interconnects;and

FIGS. 5A-5D are illustrations of embodiments of BoT interconnects.

DETAILED DESCRIPTION

The making and using of the presented embodiments are discussed indetail below. It should be appreciated, however, that the presentdisclosure provides many applicable concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the describedpackage, and do not limit the scope of the disclosure.

Embodiments will be described with respect to a specific context, namelymaking and using bump-on-trace interconnects useful in, for example,package-on-package assemblies. Other embodiments may also be applied,however, to other electrically connected components, including, but notlimited to, bare chips, displays, input components, board mounting, dieor component mounting, or connection packaging or mounting ofcombinations of any type of integrated circuit or electrical component.

The embodiments of the present disclosure are described with referenceto FIGS. 1 through 5D, and variations of the embodiments are alsodiscussed. Throughout the various views and illustrative embodiments ofthe present disclosure, like reference numbers are used to designatelike elements. Additionally, the drawings are intended to beillustrative, are not to scale and not intended to be limiting. Notethat, for simplification, not all element numbers are included in eachsubsequent drawing. Rather, the element numbers most pertinent to thedescription of each drawing are included in each of the drawings.

The present concepts are directed to providing a system and method forcreating interconnections having a solder based bump-on-trace (BoT)connection with an improved pitch. Additionally, the disclosedembodiments are not limited to bump-on-trace applications, but may beapplied to lead grid arrays (LGAs) where an array of conductivestructures protrudes from a package for attachment to another package.LGA leads may be formed to have flexibility to absorb thermal orphysical stress in a package-on-package connection, and solder may beapplied to a portion of each LGA lead to attach the lead to a trace orbump.

With BoT connectors having fine pitches (<100 μm), the bump solder tendsto not wet the sidewall of a trace under a thermal compressionbonding/nonconductive paste (TCB/NCP) process, negatively impactingjoint integrity and electro-migration performance. BoT interconnectsystems may provide a higher density of interconnects than alternativemethods of packaging, and reduce the failure rate of interconnectedassemblies. BoT interconnects may be used to attach, or stack multiplepackages vertically, connecting the stacked packaged via redirectionlayer (RDL) contacts, electrical traces, mounting pads or the like.

In general terms, in the illustrated embodiments, a BOT joint canachieve fine pitch assembly with significant trace sidewall solderwetting. In one embodiment, one or both of the trace sidewalls may bewetted, or covered, by solder, at more than half the trace height.Sidewall wetting may provide for advantageous features that include, butare not limited to, improved joint integrity (e.g., reduced tracepeeling and TO joint cracking) and improved electro-migration (EM)performance.

Referring to FIG. 1, a cross-sectional diagram of an embodiment of apackage 100 with a sidewall wetting BoT interconnect is depicted. Apillar or bump 122 may be disposed on a first substrate 102 or onanother package. In one embodiment, the bump 122 may be copper (Cu), orit may be gold (Au), silver (Ag), palladium (Pd), tantalum (Ta), tin(Sn), lead (Pb), alloys of the same, or another conductive material.Additionally, the bump 122 may have an adhesion or anticorrosion coatingsuch as an organic solderability preservative (OSP), tin (Sn),nickel-gold alloy (NiAu), nickel-platinum-gold alloy (NiPtAi) or thelike. While the top package is herein referred to as a first substrate102, in other embodiments, a flip chip, a display, package, PCB, inputboard, or any other electronic device may be used within the scope ofthe present disclosure. Additionally, while a single level ofinterconnects are described herein, the present disclosure may beapplied to a system having multiple packages on any number of levels orin any arrangement. For example, a memory chip may be mounted on aprocessor using a sidewall wetting BoT interconnect, and theprocessor/memory combination may be mounted to a PCB or other targetpackage using the sidewall wetting BoT interconnect technique disclosedherein.

In one embodiment, a first substrate 102 may be a chip, package, PCB,die, or the like, and may have a die substrate 104 and one or moremetallization layers 106. The metallization layer 106 may, in oneembodiment, include a conductive land 108, metallic traces, vias, or thelike. An oxide or insulating layer 110 and passivation layer 112 mayeach optionally be disposed on the surface of the first substrate 102,and may define an opening over the conductive land 108 for the bump 122to contact or attach to the conductive land 108. In such an embodiment,the bump 122 may be disposed covering a portion of, or the entireexposed portion of, the conductive land 108 not covered by theinsulating layer 110 and passivation layer 112. Additionally, the bump122 may be disposed to cover or contact a portion of the insulationlayer 110 or passivation layer 112. In such an embodiment, the bump 122may be disposed over the conductive land 108 and the insulating layer110 or passivation layer 112. In some embodiments, the bump 122 may becompletely cover the conductive land 108 and contact the insulatinglayer 110 or passivation layer 112 on all sides of the conductive land108 to seal the conductive land 108 from the environment.

The first substrate 102 may be electrically coupled to a die substrate114 disposed in the second substrate 120 and having a conductive trace116 formed thereon. The trace 116, in one embodiment, may be depositedas a blanket surface and patterned, but in other embodiments, may beformed via a damascene process, or the like. Additionally, the trace 116may, in one embodiment, be copper, or another conductive material, andmay optionally have an anticorrosion coating such as an OSP, metalliccoating or the like.

Application of a conductive material 124, such as solder, may assist inretaining the electrical connection between the bump 122 and the trace116. Solder joints avoid electromigration issues, and the use ofsidewall wetting creates a stronger joint at the solder joint 124 totrace 116 junction. Such sidewall wetting may prevent cracking of thejoint, or delamination of the solder joint 124 from the trace 116, duein part to an increased surface area, but also due to the materialwetting the trace 116 sidewall preventing lateral movement of the solderwith respect to the trace 116.

Thermal compression bonding is the welding of two metallic structuresusing heat and pressure. However, imperfections, such a surfaceirregularities, oxidation or contaminants on the mating surfaces maycreate voids when two surfaces are brought together for bonding.Electromigration exists where the flow of electrons in a metal causesatoms to move due to the electrons striking the atom and transferringthe electrons' momentum to the atom. EM is a particular problem in smallPCB joints due to the grain boundary of the like metals forming thejoints, as the migration of metal atoms tends to occur around any voidsor impurities in the interface between the two structures forming thejoint. This atom migration amplifies the imperfections in the joint,eventually leading to physical failure of the joint.

In one embodiment a conductive material is used to form a mechanical andelectrical connection between the bump 122 and trace 116. In someembodiments, the conductive material may be solder; however, anotherfusible conductive material may be used, such as, but not limited togold, conductive adhesive, solder paste, or the like. The illustratedconfiguration illustrates one embodiment with wetting of the sidewallsof the trace 116, which will preferably be at least half the height ofthe trace 116 sidewall. In another embodiment, the sidewalls of thetrace 116 will have solder disposed on, or wetting, at least a portionof one trace 116 sidewall. The wetting may be promoted by treating thetrace 116 sidewall to more readily accept the solder. In someembodiments, an active plasma treatment may be applied to prepare thesurface for application of the solder joint 124. In another embodiment,the trace 116 sidewall may be chemically treated, for example, to removeoxidation or foreign material from the surface of the trace. However,wetting may be promoted by any process, including surface etching,applying a flux, applying a solder preservative, or the like.

Additionally, the region of the trace 116 sidewall wetted by the solderjoint 124 will be a contiguous portion of the solder joint 124, with theentirety of the solder joint 124 being applied or formed in a singlestep. For example, the solder joint 124 may be reflowed and solidifiedto create a uniform structure over the trace 116. In another embodiment,the solder joint 124 may extend past the face, or surface of the bump122 opposite the first substrate 102, and may cover a portion of asidewall of the bump 122.

The embodiment illustrated in FIG. 1 shows bump 122 having substantiallyvertical sidewalls. Skilled artisans will recognize that the bump 122may have a sidewall slope that varies depending on the requirements ofthe joint. For example, the bump 122 may have sloped sides, or may havea curved or partially circular vertical cross section. Additionally,while the foregoing examples are described in terms of the verticalcross-section, the bump 122 may have virtually any advantageoushorizontal cross section. For example, the bump 122 may have a round,square, rectangular or irregularly shaped base in combination with anyvertical cross section.

FIG. 2 depicts a cross-sectional diagram of an alternative embodiment ofa package 200 with a sidewall wetting BoT interconnect. In such anembodiment, the sidewalls of the bump 122 may be sloped, with thebroader, or wider, portion of the bump 122 being disposed at the firstsubstrate 102, and the narrower end having the solder joint 124 disposedthereon. The wider end of the bump may be disposed on the firstsubstrate 102 conductive land 108 and covering a portion of theinsulating layer 110 and passivation layer 112. In such an embodiment,the trace 116 may have a portion of the solder joint 124 wetting, ordisposed on, a portion of the sidewalls.

FIG. 3 is an enlarged cross-sectional illustration 300 of a BoTinterconnect. Use of a wetted sidewall trace BoT joint may also permit afiner pitch between adjacent interconnects structures. FIG. 4 iscross-sectional illustration of multiple BoT interconnects. In theembodiments shown in FIGS. 3 and 4, the bump 122 has sloped sidewalls,however, a parallel or straight sided bump 122, as illustrated in FIG.1, or any other described or useful bump shape may be used as well.

Referring now to FIG. 3, the bump face 310 may have a bump face width312 that may be wider than the trace width 314. The solder joint 124 mayhave a width greater than the trace width 314. The portion of the solderjoint 124 exceeding the trace width 314 may extend below the face of thetrace to wet the sidewall of the trace 116. Additionally, in someembodiments, the width of the solder joint 124 may exceed the bump facewidth 312 to wet the sidewalls of the bump 122. In another embodiment,the width of the solder joint 124 may be about equal to the bump facewidth 312 and wet the sidewalls of the trace 116. In yet anotherembodiment, the bump face 310 may be disposed above and separate fromthe trace 116 by a predetermined joint gap distance 308 that issufficient to permit solder to reflow through the gap without voids andresult in a predetermined joint height.

The sidewall height 302 is comprised of the sidewall wetted regionheight 304 and the sidewall unwetted region height 306. In oneembodiment, the sidewall wetted region height 304 may be at least halfof the sidewall height 302. In another embodiment, the sidewall wettedregion height 304 may be equal to the sidewall height 302, that is, theentire trace 116 sidewall may be wetted by the solder joint 124.

In one embodiment, the joint gap distance 308 may be the same as theheight of the trace, or sidewall height 302. In another embodiment, thejoint gap distance 308 may be less than the sidewall height 302 of thetrace 116. Therefore, the joint gap distance 308 may be sufficient topermit solder to flow into the gap, and less than the sidewall height302 of the trace 116.

Referring now to FIG. 4, two bumps 122 are separated by a predetermineddistance, that determine the bump pitch 408 in combination with theoverall width of the bump 122 and related structures making up a singleBoT interconnect. In some embodiments, an adjacent trace 414 may bedisposed near a BoT interconnect, and separated from a bump 122 by abump-to-trace separation width 416. In some embodiments, the adjacenttrace width 412 may be narrower than trace width 314. However, theadjacent trace is not limited to having a narrower width, as anydimension of adjacent trace 414 may be used.

The bump pitch 408 is the distance between like elements on adjacentstructures, and is comprised of the bump separation distance 402 and thebump width 410, and in one embodiment, the bump pitch 408 may be about140 μm or less. For the bumps 122 illustrated here, the minimum bumppitch 408 may be determined at least partly by the bump width 410, butalso by the solder joint separation width 404 and bump-to-traceseparation width 418. The trace separation distance 406 is determined bythe bump separation distance 402 in combination with the differencebetween the trace width 314 and the bump width 410. The solder jointseparation width 404 will, in one embodiment, be greater than the bumpwidth 410. This results in a conductive material joint having a widthless than the bump width 410.

The solder joint separation width 404 will, in one embodiment, begreater than the bump width 410. In an embodiment with a bump 122 havingtapered sidewalls, the solder joint 124 may have a width less than thewidth of the widest part of the bump 122, or the bump width 410illustrated herein, and may simultaneously have a width greater than thebump face width 312. Additionally, the solder joint 124 may have a widthless than the bump width 410.

The width of the solder joint 124 may be determined by the volume ofsolder applied to form the solder joint 124. In one embodiment, thevolume of solder required to form a solder joint 124 having apredetermined width and trace sidewall wetted region height 304 may bedetermined by the joint gap distance 308, solder joint separation width404, bump-to-trace separation distance 416, trace 116 geometry, adjacenttrace 414 geometry, and bump 122 geometry. In one embodiment, the volumeof solder forming the solder joint 124 will be sufficient to wet thetrace 116 sidewalls to a desired height and still provide a solder jointseparation width 404 sufficient to prevent bridging of a solder joint124 to an adjacent solder joint 124 or connection structure.

A method for forming a wetted sidewall trace BoT joint may, in oneembodiment, comprise providing a first substrate 102 or other substrate,and forming one or more bumps 122 on the first substrate 102. The volumeof a conductive material, such as solder, required for a predeterminedwidth of solder joint 124 may optionally be calculated or optimizedusing joint parameters including, but not limited to one or more of thejoint gap distance 308, a predetermined or desired solder joint width, apredetermined solder joint separation 404, the bump 122 geometry, thetrace 116 geometry, the minimum trace 116 sidewall wetting region heightor trace separation distance 406. The solder joint 124 may be applied inthe calculated volume to the bump 122 as a solder cap.

The first substrate 102 may be singulated or removed from a wafer,singly or in predetermined first substrate 102 strips or groups, and mayhave final packaging steps performed. A second substrate 120, such as aPCB, chip, package, die, or the like, may be created by placing one ormore traces 116 on a die substrate 114, and the first substrate 102 maythen be placed on the second substrate 120, with the bump 122 andapplied solder caps aligning with traces 116 on the second substrate120. The assembly of the first substrate 102 and second substrate 120may be heated for reflow to a temperature where, preferably, the solderreaches at least a eutectic point such that the solder melts orsolidifies in a single step, without intermediate phases. The firstsubstrate 102 may be moved towards or held apart from the secondsubstrate 120 at a predetermined distance during reflow so that the bumpfaces 310 are about a predetermined joint gap distance 308 above thefaces of the traces 116, and so that the solder of the solder bump wetsthe sidewall of the trace 116 to cover about a predetermined portion ofthe trace 116 sidewall.

FIG. 5A illustrates another embodiment of a BoT joint. In such anembodiment, the solder joint 124 may wet the sidewalls of the trace 116,while not wetting the sidewalls of the bump 122. Alternatively, asillustrated in FIG. 5C, the solder ball may wet one side of the bump122, and not the other. This may be the result of the solder joint 124forming a bulge 502 where the solder fails to flow along the bump 122sidewall. This may also be a result of the trace 116 being offset fromthe bump 122. Thus, the BoT may correct for accidental or intentionalmisalignment of the trace 116 and bump 122, permitting packages withmismatched mounting arrangements to be joined.

FIG. 5B illustrates an embodiment of a BoT joint with an alternativebump. In such an embodiment, the bump face width 312 may be greater thanthe base of the bump 122 having bump width 410. The solder joint 124may, in such an embodiment, wet the sidewalls of the bump 122, furtherstrengthening the attachment of the solder joint 124 to the bump 122 bymechanical means.

In accordance with an embodiment, a method includes providing a firstsubstrate having a bump disposed thereon, and the bump having a volumeof conductive material disposed thereon. The method further includesproviding a second substrate having a conductive trace, the conductivetrace having a sidewall. The method further includes mounting the firstsubstrate on the second substrate. The mounting resulting in anelectrical connection from the bump to the conductive trace. The bump isseparated from the conductive trace by a distance less than a height ofthe conductive trace, and the conductive material is at least partiallycovers a sidewall of the conductive trace.

In accordance with another embodiment, a method includes disposing asolder joint on a bump electrically connected to a conductive land in afirst substrate. A first surface of the bump distal to the conductiveland has a first width. The method further includes aligning the firstsubstrate to a second substrate by aligning the solder joint to aconductive trace of the second substrate. The method further includesreflowing the solder joint to bond the solder joint with the conductivetrace. The solder joint at least partially wets sidewalls of theconductive trace. A lateral surface of the conductive trace contactingthe solder joint has a second width less than the first width.

In accordance with yet another embodiment, a method includes disposing abump on a first package component. The first package component includesa die substrate, a conductive land over the die substrate, and adielectric layer over and covering edges of the conductive land. Thebump is disposed over and electrically connected to the conductive land,and a surface of the bump opposite the conductive land is substantiallylevel, and a sidewall of the bump is substantially straight in across-sectional view of the bump. The method further includes disposinga solder ball on the surface of the bump opposite the conductive land;and bonding the first package component to a second package component.After bonding the first package component to the second packagecomponent, a portion of the solder ball is disposed on a sidewall of aconductive trace of the second package component.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure that processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

What is claimed is:
 1. A method of forming a semiconductor package, themethod comprising: receiving a first semiconductor package, the firstsemiconductor package comprising a first substrate and a conductivepillar at a first side of the first substrate; receiving a secondsemiconductor package, the second semiconductor package comprising asecond substrate and a conductive trace on a first surface of the secondsubstrate, a sidewall of the conductive trace having a first height; andbonding the conductive pillar to the conductive trace using a conductivejoint, wherein after the bonding, the conductive joint covers thesidewall of the conductive trace by at least half the first height, andthe conductive pillar is spaced from the conductive trace by a firstdistance, the first distance being smaller than the first height.
 2. Themethod of claim 1, wherein the conductive joint comprises solder.
 3. Themethod of claim 2, wherein bonding the conductive pillar comprisesperforming a reflow process to bond the conductive pillar to theconductive trace.
 4. The method of claim 1, wherein a first surface ofthe conductive pillar distal from the first substrate has a first width,and a second surface of the conductive trace distal from the secondsubstrate has a second width, wherein the first width is larger than thesecond width.
 5. The method of claim 1, wherein a width of theconductive pillar, measured between opposing sidewalls of the conductivepillar, changes continuously along a first direction from the first sideof the first substrate to a first surface of the conductive pillardistal from the first substrate.
 6. The method of claim 5, wherein thewidth of the conductive pillar increases along the first direction. 7.The method of claim 1, wherein the conductive joint is formed to extendfrom the conductive trace to a first surface of the conductive pillarfacing the conductive trace, and no conductive joint is formed onsidewalls of the conductive pillar.
 8. The method of claim 1, whereinthe conductive joint is formed to extend from the conductive trace to afirst surface of the conductive pillar facing the conductive trace, andis formed to extend along a first sidewall of the conductive pillar. 9.The method of claim 8, wherein the conductive joint is formed to extendalong a second sidewall of the conductive pillar opposing the firstsidewall.
 10. The method of claim 8, wherein no conductive joint isformed to extend along a second sidewall of the conductive pillaropposing the first sidewall.
 11. The method of claim 10, wherein aperipheral area of the first surface of the conductive pillar is exposedby the conductive joint.
 12. The method of claim 10, wherein theconductive joint has a bulge proximate to the second sidewall of theconductive pillar, wherein the bulge extends beyond the second sidewallof the conductive pillar.
 13. A method of forming a semiconductorpackage, the method comprising: positioning a conductive pillar of afirst package over a conductive trace of a second package, wherein afirst end of the conductive pillar is attached to a conductive featureof a first substrate of the first package, and a second end of theconductive pillar extends away from the first substrate, wherein theconductive trace is disposed along a surface of a second substrate ofthe second package, and sidewalls of the conductive trace have a firstheight; and forming a solder region to bond the conductive pillar to theconductive trace, where after the solder region is formed, theconductive pillar is spaced apart from the conductive trace by adistance smaller than the first height, and the solder region wets uppersidewalls of the conductive trace by a second height, the second heightbeing larger than half of the first height.
 14. The method of claim 13,wherein the second end of the conductive pillar is formed to have afirst width larger than a second width of the conductive trace.
 15. Themethod of claim 13, wherein the solder region is formed to wet at leasta portion of a first sidewall of the conductive pillar.
 16. The methodof claim 15, wherein a second sidewall of the conductive pillar opposingthe first sidewall is free of the solder region.
 17. The method of claim16, wherein a portion of the second end of the conductive pillar is freeof the solder region.
 18. A method of forming a semiconductor package,the method comprising: aligning a first semiconductor device with asecond semiconductor device, wherein the first semiconductor devicecomprises a first substrate, a conductive land at a first side of thefirst substrate, and a conductive pillar coupled to the conductive land,wherein the second semiconductor device comprises a second substrate anda conductive trace on a surface of the second substrate facing theconductive pillar, a sidewall of the conductive trace having a firstheight; and bonding the conductive pillar to the conductive trace byforming a solder joint in between, wherein the solder joint covers thesidewall of the conductive trace by at least half the first height, andthe conductive pillar is spaced apart from the conductive trace by afirst distance, the first distance being smaller than the first height.19. The method of claim 18, wherein the solder joint covers at least aportion of a first sidewall of the conductive pillar, and a secondsidewall of the conductive pillar opposing the first sidewall is free ofthe solder joint.
 20. The method of claim 19, wherein an area of an endsurface of the conductive pillar facing the conductive trace is free ofthe solder joint.